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Offset + gain on single opamp?
I've been playing around with getting both gain and offset on a single non-
inverting op amp and I'm not quite getting how to calculate the offset input voltage needed to get 1.27 volts out. I'm hoping someone might know what I'm missing. What I attempted was to get 2.5x gain with a +1.25 volt offset - so a zero volt input on the non-inverting input would give 1.25 volts out while a 1 volt input would give 3.75 volts out. At first, I thought that since there was to be a 2.5x gain, I needed to add -1.25/2.5= -0.5V to the inverting input. I used a voltage divider to create the -0.5V. from the -5V supply. Next I went to tackle the gain. I calculated the thevenin equivilent resistance of the voltage divider then calculated the size of the feedback resistor and put it all together on a breadboard. I got the gain right - but with 0V in I get 0.750V out - which is only 1.5 times the offset voltage I supplied. Ok, so my assumption about the affect of gain on the supplied offset voltage was wrong - I went looking on google to see if I could find out how to calculate the voltage required: two days ago! To sum up: I'm using an LMC6482 CMOS amp with 10 teraohm input impedance. Input signal is a 10k resistor to either ground or a voltage divider putting out 1V. I can calculate the gain with Thevenin's but I cannot figure out how to calculate the offset voltage required. Anyone able to steer me in the right direction? P.S. I have this working with two opamps (one chip). Its just bugging me that I cannot figure out how to do it with a single stage. |
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